Capped film resistors



Oct. 13, 1964 F. G. STREET 3,153,219

CAPPED FILM RESISTORS Filed Sept. 5, 1961 F .1 I2 30 I3 "7 30 i .Z IO 2Z1 33 A m V -I'B" 13- 2:13- 3..

ATTORNEYS stool; and then cutting United States Patent 3,153,219 CAPPED FILM RESES'EGRS Frederick G. Street, Bradford, Pa, assignor, by mesne assignments, to Air Reduction (Iompany, Incorporated, a corporation of New York Filed Sept. 5, 1961, Ser. No. 136,911

v Claims. ((31. 338-273) This invention relates to capped film resistors. Such resistors are commonly manufactured from rod stock. Normally, film-type resistors are made using substrates cut to length and then applying the film of resistive inaterial to the substrates or by applying the film to the rod the desired length. Caps are then placed over the ends of the substrate with conductors leading from'thc caps away from the substrate. The assembly is then usually coated with electrically insulating material.

It is an object of this invention to provide an improved capped film resistor of the character indicated, and to provide an improved method for making such resistors.

More specifically, it is an object of the invention to provide resistors having the end portions of the substrates of reduced diameter and gripped by caps which have an outside diameter not substantially larger than the outside diameter of the substrate. Such resistors have the important advantage of having substantially uniform diameter throughout their length and this greatly facilitates the handling of the resistors by automatic machines.

The construction of this invention has the additional advantage that ordinary commercial rod stool: can be used to make the substrates and the end portions, which are to gripped by the caps, can be reduced to uniform diameter to close tolerances even though the commercial rod stock, which is usuallyglass or a ceramic, has wide diameter tolerances.

The capped film resistor of this invention can also be made with the outside diameter of the cap somewhat less than the diameter of the substrates, and with a molded coating of electrically insulating material of uniform outside diameter and of slightly greater radial thickness over the caps.

Another advantage of the invention is that the reduceddiameter portions of the stock used for the substrates provide a shoulder against which the caps seat while the center portions of the caps are spaced from the broken ends of the substrates. This provides more uniform dimensions for the finished resistors.

Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.

In the drawing, forming a part hereof, in which like 'eference characters indicate corresponding parts in all the views;

FEGURE 1 is a'diagrammatic view showing one step in the manufacture of the film resistors of this invention;

FEGURE 2 is a fragmentary View, partly in section, showing the full-diameter portions of the stock of PEG- URE 1 coated with a film of resistive material;

FIGURE 3 shows a modified construction in which the resistive material is applied to the entire surface of the stock after portions of the stock have been turned down; i

FlGURE 4 is an enlarged sectional view of a capped film resistor made in accordance with this invention;

FIGURE 4a is a fragmentary view, similar to FIG- URE 4, but showing a construction or" the prior art;

FIGURE 5 is a sectional View on the line 5-5 of FIGURE 4;

lGURE 6 is a view similar to FIGURE 4 but showing a resistor made from tube stock;

Patented Oct. 1 3, 19fi4 FIGURE 7 is a sectional view, to reduced scale taken on the line 7-7 of FlGURE 6; 7

FIGURE 8 is a fragmentary view of another modified form of the invention; and

FIGURE 9 is a fragmentary sectional view similar to FIGURE 6, but showing another modified form of the invention.

FIGURE 1 shows a portion of a length of rod stock it? having full-diameter parts 12, 13 and 4 spaced from one another by regions where the stock has been turned down to reduced diameters. There is a circumferential groove to midway between each of the full-diameter parts of the stock. On opposite'sides of the circumferential grooves 16 there are end portions. 13 which are of reduced diameter so as to leave a shoulder 2t) where these end portions 13 meet the full-diameter parts 12, 13

and 14.

The stock to is broken apart at the circumferential grooves in. Each of the separated parts thus becomes a substrate for a film resistor. if the stock is to be cut instead of broken, the grooves to? are not necessary.

A film of resistive material 22 (FIGURE 2) may be applied to the stock ltl before regions of the stock are turned down. When such is the case, there is no resistive material over, the end portions 18 since the resistive material is removed by the turning-down operation.

FIGURE 3 shows a modified construction in which resistive material 22' is applied to the rod stock after it has been turned down. In this case the resistive material covers the end portions 18.

The circumferential grooves in and the end portions 18 may be turned down by any suitable machining operation which is effective for glass or ceramic or other electrically insulating material from which the substrates are made. Glass is commmonly used and the turning down is done by grinding. Various ceramic stocks can be used. Some of these require grinding others can be machined by other methods.

FIGURE 4 shows, to somewhat larger scale, a capped film resistor made from one of the substrates formed from the stock 1d. The substrate is indicated by the reference character ill. This substrate has the resistive material 22 covering both the full-diameter parts of the substrate or body of the resistor and also covering the reduced-diameter end portions 13.

A metal cap 26 is pressed over each end portion 18. This metal cap has an inside diameter slightly less than the diameter of the end portion 18 so that some force is required to press the cap 26 over the end portion 18, and the cap grips the endportion with substantial friction. The cap 26 is connected to a conductor 28. In the construction illustrated, the conductor 28 has a shoulder 29 a short distance back from the end of the conductor. The endot the conductor is inserted through a closely fitting hole in the cap and the shoulder 29 stops further movement of the conductor into the cap." The inwardly projecting end of the conductor 28 is then swaged to form a head 36 inside the cap and to secure the cap to the conductor. Sometimesthe conductor 28 is soldered to the cap and sometimes a conductor is Welded to an imperforate cap. The construction shown is merely representative of Wire-attached caps.

In the construction illustrated, the side walls of the caps 26 are cylindrical, but these caps are sometimes made polygonal in cross section. Where polygonal side walls are used, it is common practice to use an octagonal shape and to have each of the sides oftheoctagon bend outwardly to some degree where the cap grips the cylindrical end portion of the substrate. Where the description of this invention refers to diameter of the cap, the expression is not intended to be limited to cylindrical side Walls and the inside and outside diameters of polygonal side Walls are to be considered as the diameters of the inscribed and circumscribed circles, respectively, for the particular polygon section.

The difference between the diameters of the end portions 18 and the full-diameter portion of the substrate is at least as great as the difference betwen the inside and outside diameters of the side wall of the cap 26. This means that the outside diameter of the cap 26 is substantially flush with the full-diameter part of the substrate, or is somewhat lower than this full-diameter part. In the construction shown in FIGURE 4, the outside circumferential surfaces of the caps 26 are flush with the circumferential surface of the substrate Ill between the caps.

The fit of the caps on the substrate is very important. If they are not tight they do not provide good contact with the resistive material. Caps with cylindrical side walls usually get firm contact at not more than three points since the confronting cylindrical surfaces are not perfect circles. A cap with an octagonal side wall gripping a cylindrical end of a substrate usually obtains eight points of firm contact.

An insulating coating 34 is usually applied over the assembly. This coating extends for the full length of the assembly and the end portions of the coating 34 preferably cover most of the end faces of the caps as.

FIGURE 4a shows a construction of the prior art in order to bring out the differences between this construction and that shown in FIGURE 4. A substrate 35 is covered with a film of resistive material 22 but the substrate is of uniform diameter throughout its entire length. A cap 35 grips the end of the substrate but produces an increase in diameter equal to twice the Wall thickness of the cap, so that when the resistor is covered with insulating material 37 there is a change in the diameter of the insulating material at the region 38 where a sloping shoulder is formed on the outside surface of the resistor.

Such a change in diameter also occurs at the other end of the resistor and this non-uniform outside surface of resistors has caused great problems in using the resistors in automatic assembly equipment. One disadvantage has been that because of the fact that the resistors do not have a cylindrical outside surface, they do not roll uniformly in chutes which are used with automatic equipment. An even more serious disadvantage has been that the jaws of automatic assembly equipment do not always grip the resistor accurately and when the jaws contact with a resistor near one of the sloping shoulders 38, it has been a common experience to have the resistor shift in the grip of the jaws so that it is not in the proper orientation when presented to the other part of the assembly with which it is to be connected.

A comparison of FIGURE 4 and FIGURE 4a brings out clearly the difference in the outside contour of the resistor which is obtained by having the end portions of the substrate of a reduced diameter so as to have the outside of the cap substantially flush with the cylindrical surface of the resistive material on the substrate.

FIGURE 6 shows a modified construction in which a substrate 40 is made of tube stock instead of rod stock. Other parts which are identical with the other figures are indicated by the same reference characters as in the other figures. The construction of FIGURE 6 also differs from FIGURE 4 in that the resistive material 22 is also applied to the stock before turning down the end portions 13, this feature being similar to FIGURE 2.

In order to maintain a continuous circuit from each of the caps 26 to the resistive material 22, an electrically conducting material 42, usually a silver material, is applied to the shoulders 2d and this conducting material P preferably extends all the way along the outside surface of the end portion 18 which is gripped by the cap, and lays over the ends of the areas that are covered by the resistive material 22. In the preferred construction the material 42 is a bonding material which co-operates i with the friction grip of the cap 26 in holding the cap on the end portion 18 of the substrate. Such bonding material can also be used with the constructions shown in FIGURES 4 and 8, if desired.

The substrate 46 is preferably made with an annular end face 44 at which the substrates are broken apart after machining circumferential grooves at the locations where the tubular stock is to be broken. Bevel faces 46, which are sides of the circumferential grooves, are left at both ends of the substrate 40. However, tubular substrates can be made by cutting circumferential grooves through the full radial thickness of the tubular stock so that there is no annular end face 44 but only a bevel face 46.

FIGURE 8 shows another modified construction which is similar to FIGURE 6 except that the reduced end portion, designated by the reference character 18', is of such small diameter that the outside diameter of a cap 26 is less than the full diameter of a substrate 40'.

A coating 34 is molded, rolled on, sprayed on, or otherwise applied over the assembly in FIGURE 8 with the outside diameter of the coating 34' uniform throughout the length of the assembly. This makes the radial thickness of the coating 34 greater over the cap 26' than it is over the full-diameter part of the substrate. The uniform outside diameter of the resistor is thus maintained, and the greater thickness of the coating 34', toward the ends of the resistor, provides greater insulation resistance at regions which are more subject to dielectric failure.

FIGURE 9 shows another modified construction in which a substrate 5% is tubular and similar in construction to substrate 4% shown in FIGURE 6, except that there is resistive material 52 on the inside surface of the tube instead of on the outside surface. Electrically conductive material 42 overlaps the end portion of the resistive material 52 and extends around the end faces of the substrate tube St and across the periphery of the reduced diameter portion of the substrate.

A cap as made of metal, fits over the electrically conducting material 42 and tightly grips the reduced diameter portion of the substrate 59 in the same way as the caps shown in the other figures. A conductor 23 is attached to the cap 56 in the same way as already described for the conductor 28 in the other construction; and there is a layer of insulation 34 covering the resistor of FIGURE 9, and which is also similar to the constructions shown in FIGURES 4 and 6.

While all of the resistors illustrated in the drawing have coatings of resistive material which extends along the length of the substrate without interruption in the continuity of the resistive material, resistors of this type are sometimes made with the resistive material in the form of a helix so that the path which must be followed by the electric current is much longer and the resistance is proportionately higher. This invention can be applied equally well to resistors having a film of resistive material which does not cover the entire cylindrical surface of a substrate; and the resistive material can be made to cover a helical area or other shape of area by scraping off some the resistive material after the entire surface has been coated. For making a helical area, the resists may be removed by a simple machining operation similar to the cutting of a thread on a lathe.

The preferred embodiments of the invention have been illustrated and described, but changes and modifications can be made and some features can be used in different combinations without departing from the invention as defined in the claims.

What is claimed is:

l. A metal oxide electrical resistor including a onepiece, solid core made of electrically insulating material and of circular cross section, the core having a full and uniform diameter throughout most of its length and having relatively short end portions that taper to a smaller diameter and then form circumferential surfaces of uniareas form diameter to end faces of the core, each of the end faces of the core merging with the adjacent smaller diameter portion around a beveled corner, metal oxide electrical resistance coating of substantially uniform thickness covering the full diameter, converging, reduced-diameter and beveled-corner surfaces of the solid core, metal end caps that fit over the resistance coating on the reduceddiameter portions of the core with a press fit whereby said caps are secured to the opposite ends of the core by friction, the wall thickness of the parts of the caps that fit over the reduced-diameter portion of the core being substantially equal to the difference in the radius of the full diameter and reduced-diameter portions of the core, electrically insulating material covering the entire outside surfaces of the caps and the coated core between the caps, the electrically insulating material being of substantially uniform outside diameter, and an electrical terminal conductor connected to the end of each cap adjacent to the axis of the core and extending through and beyond the insulating material that covers the cap.

2. A capped electrical element including a one-piece body of electrically insulating material and having end portions of limited axial length and of smaller diameter than that of the body between the end portions, metal caps having side walls that fit over the end portions of the body, the dilterence between the diameter of the end portion and that of the body between the end portions being of a dimension substantially equal to the difference be ,tweenthe inside and outside diameters of the side Walls of the caps, electrically conductive material supported by the body and electrically connected with the metal caps,

- and a continuous coating of electrical insulation covering the outside of the body and the side Walls of the metal caps, said insulation being of substantially uniform outside diameter over both the caps and the portion of the body between the caps.

3. The capped electrical element described in claim 2 characterized by the body being of circular cross section throughout its length, the caps having end faces over the ends of the body, the coating of electrically insulating material covering the ends of the caps, and electrical terminal connectors joined to the end faces of the caps and extending outward through and beyond the insulation.

4. The capped electrical element described in claim 3 characterized by the electrically conductive material on the body being a resistance coating and being continuous along the full diameter part of the body and to and across parts of the circumferential surface of the smallerdiameter portions of the body, and the caps fitting over the electrical resistance coating on the smaller-diameter portions of the body with a press fit whereby said caps are secured to the body by a strong friction grip.

5. The capped film resistor described in claim 4 and in which the body is a tube with the walls thinner and of reduced outside diameter along the end portions and with annular end faces and beveled corners where the annular end faces meet the circumferential outside surface of the end portions.

References Cited in the file of this patent UNITED STATES PATENTS 2,271,774 Megow Feb. 9, 1942 2,282,651 Haberberger May 12, 1942 2552,64 9 Morin May 15, 1951 2,557,571 Siegel June 19, 1951 2,739,212 Woolley Mar. 20, 1956 FOREIGN PATENTS 536,832 Canada Feb. 5, 1957 

1. A METAL OXIDE ELECTRICAL RESISTOR INCLUDING A ONEPIECE, SOLID CORE MADE OF ELECTRICALLY INSULATING MATERIAL AND OF CIRCULAR CROSS SECTION, THE CORE HAVING A FULL AND UNIFORM DIAMETER THROUGHOUT MOST OF ITS LENGTH AND HAVING RELATIVELY SHORT END PORTIONS THAT TAPER TO A SMALLER DIAMETER AND THEN FORM CIRC UMFERENT IAL SURFACES OF UNIFORM DIAMETER TO END FACES OF THE CORE, EACH OF THE END FACES OF THE CORE MERGING WITH THE ADJACENT SMALLER DIAMETER PORTION AROUND A BEVELED CORNER, METAL OXIDE ELECTRICAL RESISTANCE COATING OF SUBSTANTIALLY UNIFORM THICKNESS COVERING THE FULL DIAMETER, CONVERGING, REDUCED-DIAMETER AND BEVELED-CORNER SURFACES OF THE SOLID CORE, METAL END CAPS THAT FIT OVER THE RESISTANCE COATING ON THE REDUCEDDIAMETER PORTIONS OF THE CORE WITH A PRESS FIT WHEREBY SAID CAPS ARE SECURED TO THE OPPOSITE ENDS OF THE CORE BY FRICTION, THE WALL THICKNESS OF THE PARTS OF THE CAPS THAT FIT OVER THE REDUCED-DIAMETER PORTION OF THE CORE BEING SUBSTANTIALLY EQUAL TO THE DIFFERENCE IN THE RADIUS OF THE FULL DIAMETER AND REDUCED-DIAMETER PORTIONS OF THE CORE, ELECTRICALLY INSULATING MATERIAL COVERING THE ENTIRE OUTSIDE SURFACES OF THE CAPS AND THE COATED CORE BETWEEN THE CAPS, THE ELECTRICALLY INSULATING MATERIAL BEING OF SUBSTANTIALLY UNIFORM OUTSIDE DIAMETER, AND AN ELECTRICAL TERMINAL CONDUCTOR CONNECTED TO THE END OF EACH CAP ADJACENT TO THE AXIS OF THE CORE AND EXTENDING THROUGH AND BEYOND THE INSULATING MATERIAL THAT COVERS THE CAP. 